1. Field of the Invention
The present invention relates to catalysts for producing methylamines. The catalysts have superior performances in the production of methylamines from methanol and ammonia, or through a disproportionation reaction of monomethylamine. Methylamines, including dimethylamine are used as materials for solvents such as dimethylformamide, rubber products, pharmaceuticals, surfactants, and others.
2. Description of the Prior Art
Methylamines are usually produced from methanol and ammonia in the presence of solid acid catalysts such as silica-alumina, at a temperature of approximately 400.degree. C. Alternatively, methylamines are produced through a disproportionation reaction of monomethylamine. The productions of methylamines as mentioned above produce mainly trimethylamine which is of little use from a commercial point of view. A selective production method of dimethylamine has long been earnestly desired, because demands for methylamines are directed to dimethylamine.
There are proposed a few methods for producing methylamines using zeolite catalysts which are more advantageous over the previous silica-alumina catalysts; for example, methods which use zeolite A (JP 56-69846 A), FU-1 (JP 54-148708 A), ZSM-5 (U.S. Pat. No. 4,082,805), ferrierite and erionite (JP 56-113747 A), ZK-5, Rho, chabazite and erionite (JP 61-254256 A), and silicoaluminophosphates (JP 02-734 A). They give better results than those on the basis of the thermodynamic equilibrium.
Mordenite has been investigated as a catalyst for methylamine production, among zeolite catalysts as mentioned above. U.S. Pat. No. 3,384,667 teaches mordenite, besides other zeolite catalysts (1968). JP 56-46846 A, JP 58-49340 A, JP 59-210050 A and JP 59-227841 A teach the use of natural or synthetic mordenites. JP 03-262540 A mentions the use of mordenite modified with silicon tetrachloride through a CVD method, in order to suppress formation of trimethylamine. There are various additional modifications by ion exchange or other methods.
Furthermore, JP 06-179640 A mentions mordenite wherein a ratio of silica to alumina is more than 10, while the ratio of silica to alumina is usually not higher than 10. JP 08-283207 A teaches rod form mordenite having aspect ratios of not less than 2, in respect to crystal form of mordenite.
There is not a large difference between zeolite and silica-alumina catalysts from a commercial point of view. Zeolite catalysts are superior to silica-alumina catalysts, because the former give dimethylamine, the most desired product, with a high selectivity. However, zeolite catalysts are inferior to silica-alumina catalysts, in respect to catalyst activity. During the practical catalyst life of one year, productivity of dimethylamine is at most a half of that using the silica-alumina catalyst. Thus, even though dimethylamine is produced with a high selectivity, the advantage and disadvantage are offset when compared in the practical productivities.
The present inventors previously applied patents (JP 11-35527 A, JP Application Nos. 10-187423, 10-180879 and 10-025832), based upon the findings that silica-modified silicoaluminophophates have a higher activity, along with a higher dimethylamine selectivity, when compared with well known zeolite catalysts and silicoaluminophosphates in the prior art. These catalysts were improved to provide a higher activity along with a higher dimethylamine selectivity, than silica-alumina catalysts, and also a superior dimethylamine productivity to that in the prior art (JP Application No. 10-293772). However, mordenites practically used as a catalyst for commercial production of methylamines are not comparable with silica-alumina catalysts, because of their lower dimethylamine productivity.